Now that we have `git-fetch-from-tarball' to break the cycle in commencement
for git-fetch sources, we can finally update mig to use git-fetch to allow
`--with-commit' builds.
* gnu/packages/hurd.scm (mig)[source]: Use git-fetch again.
* gnu/packages/commencement.scm (mig-boot0)[source]: Use
git-fetch-from-tarball accordingly.